Pulsed laser systems including a passively mode-locked fiber oscillator and high power fiber amplifier have been developed and are well known. In such systems, the oscillator is typically started and run at a fixed pump diode current or power based on the design of the oscillator optical cavity parameters. These oscillator cavity parameters can change with temperature or over time with variations in component parameters such as pump diode coupling efficiency. These changes can prevent the oscillator from becoming mode-locked at startup or to lose mode-locking after a successful startup. In U.S. application Ser. No. 10/813,173 of the same assignee, “Method and Apparatus for Controlling and Protecting Pulsed High Power Fiber Amplifier Systems”, incorporated by reference herein, a control system is provided to turn off the laser to prevent damage to the system in such an event.
In U.S. Pat. No. 6,693,927 to Horvath et al., a control system is described for reinitiating mode-locking when mode-locking is lost. First, an operator predetermines a base pump diode current for maintaining mode-locking in the oscillator where the gain medium is composed of a solid-state lasing material such as Nd:glass in a bulk optics cavity. If mode-locking is lost, the current is increased by a fixed percentage to overdrive the oscillator. Once mode-locking has been obtained, the current is reduced back to the base current value. The mode-locking condition of the oscillator is periodically monitored and if mode-locking is lost, the current is again increased to the fixed overdrive value in an attempt to obtain mode-locking.
In a fiber oscillator, the Horvath method will not guarantee mode-locking due to the fact that conditions such as a temperature change of the gain fiber, pump diode coupling efficiency, or saturable absorber reflectivity may require that the pump diode current be either increased or decreased. The required amount of the increase or decrease is not necessarily a fixed percentage and will change over time and environmental conditions. An overdrive circuit such as described by Horvath thus has little value since increasing the current by a fixed amount will not help reestablish the mode-locking condition. Horvath's method can also be deleterious since it may result in double pulsing or damage if the cavity power is too high. Similarly, it may be that increasing the current by a fixed percentage will not result in a sufficient pump diode power to obtain mode-locking.
Thus it is required in a fiber laser to actively monitor not only the oscillator mode-locking, but also to measure the repetition rate and output power. In a fiber oscillator it is preferable to adjust the pump diode current to keep the oscillator output power constant. The level of control needed is dependent on the design of the mode-locked fiber laser. In U.S. Pat. No. 5,627,848 “Apparatus For Producing Femtosecond And Picosecond Pulses From Mode-Locked Fiber Lasers Cladding Pumped With Broad Area Diode Laser Arrays, one design requires a current range of +/−1% stability and another design a current range of +/−10%. In U.S. Pat. No. 5,689,519, “Environmentally Stable Passively Mode-locked Fiber Laser Pulse Source” a design which requires a current stability of +/−7% is described. It is not just the small range of current stability that is the issue. It is the long term drift of the stability range either up or down.
In a fiber oscillator, it is also desirable to be able to control and vary the temperature of the pump diode such that the wavelength of the pump diode is at the optimum wavelength for absorption by the gain fiber. The temperature of the pump diode may be increased or decreased to obtain the desired wavelength. The present invention provides a means and method for detecting and controlling the conditions needed to obtain and maintain mode-locking of a fiber oscillator.